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ERV + dehumidifier ducting in a minisplit house

BullOak | Posted in Mechanicals on

I’m working on a 2400sf house (CZ4, but very nearly 3) where the owners have selected minisplits – one ductless 1st floor, one ducted w/ very short runs for each of 2nd & 3rd. So there will be no major duct lines throughout the house for heatning and AC. They’ve also chosen to go the ERV and Dehumidifier route, this being the humid southeast.

I’m having trouble getting a straight answer from anyone about how to duct these two units without a central system to tie into. Some say keep each separate with it’s own set of 4″ smooth wall flex and distribution boxes, others say combining them and essentially running the units in parallel on the same set of ductwork. My concern with the combined system is that the Dehumidifier is ~200+ CFM and the ERV will be around a quarter of that. Separate systems seems like a lot of very small ducts.


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  1. BullOak | | #1

    Typo: 3" not 4"

  2. GBA Editor
    Martin Holladay | | #2

    For information on ducting an ERV, see these two articles:

    Ducting HRVs and ERVs

    Installing a Heat-Recovery Ventilator

    I strongly urge you to reconsider the dehumidifier. There's an excellent chance that you can control indoor humidity during the winter with the ventilation system, and during the summer with the minisplits. In most cases, the dehumidifier would be unnecessary (and expensive to run).

    For more information, see All About Dehumidifiers.

  3. BullOak | | #3

    Martin -

    They want the Dehu primarily for spring/fall, when temperatures are low enough that the minisplits won't kick on but outdoor humidity is very high for weeks or months at a time. I'm curious if anyone has collected data on how long an ERV can keep indoor humidity low once the outdoor level jumps, but given that the enthalpy core isn't 100% and humidity from cooking / showering, best I can estimate is a few days before it's exchanging too-humid air with way-too-humid air.

    More typical in this region is to dump the ERV and install a ventilating dehumidifier, though that leaves the house without ventilation during the drier winter months when the dehumidifier doesn't run.

    One option might be something like the quaternity minisplit for the 1st floor ductless unit, but the Mistubishis and fujitsus have better control systems, and it would be better to have dehumidification near the 2nd-3rd floor showers.

  4. user-2310254 | | #4

    Chris B. I have an Ultra Aire ventilator/dehumidifier. It is always ventilating, which is required for air quality. The dehumidifier only kicks in when levels exceed whatever is set on the control, which is usually around 50% in my case. The dehumidifier ran a lot when first installed but rarely kicks in now that most of the construction-related trapped moisture is gone.

  5. BullOak | | #5

    Steve- any issues with positive-pressure-only ventilation? How tight is the house?

  6. charlie_sullivan | | #6

    There are two different shoulder season dehumidification scenarios. One is when it's cool enough outside that the dewpoint is in the 50s, and ventilation can address the indoor humidity, but it can require a lot of airflow to flush it out. That's when an HRV has the biggest advantage over an ERV, as you can take care of the humidity with less airflow. Ventilation without heat recovery also works.

    The other scenario is dew points in the high 60s with temperatures in the low 70s. That's when a dehumidifier makes the most sense. There's not much else you can do except hope those conditions change.

    The ability to ride through that kind of bad condition is better than you might think, because walls, furniture, and books all buffer humidity. And if you have been cooling with minisplits, you have done better at keeping the humidity low during the cooling season, so you are going into that season well prepared.

  7. user-2310254 | | #7

    Chris. My house is 1.1 ACH/50, so it is pretty tight. My rater seemed to think the positive pressure would help with dust control by keeping it behind the drywall. I'm not sure it's actually working that way, however. There always seems to be plenty of dust even two years post-construction. Otherwise, I have not noticed any problems or issues.

  8. charlie_sullivan | | #8

    To answer your original question, my hunch is that you can get away with relatively little ducting of the dehumidifier--maybe just a single supply and a single return, preferably separated substantially If you have humidity lower in one room than another, that doesn't lead to the same kind of comfort issues that you have with temperature variations between rooms, so it's less critical. But that's just my hunch--I don't have any data, analysis, or experience to back it up.

  9. user-2310254 | | #9

    Chris. You might find this link helpful. Scroll to page 14 for a diagram and some guidance on how to duct the unit. (This link is just for informational purposes. You might need to bigger or smaller unit for your client's house.)

  10. badgerboilerMN | | #10

    Ultra-aire has the answers and you are on the right track. Mini-splits don't "automatically" de-humidify, especially when they are not running!

    I am in the process of designing/ redesigning my own shop/studio that has be stuck at 70% humidity since May. Make a place real tight, insulate the heck out of it and find yourself humid...even here in sunny Minnesota. 95F outside, 72F inside, still 70% humidity...

  11. user-2310254 | | #11

    Morgan. Have you identified any obvious moisture sources? Are you using minisplits? Just curious why your humidity is so high.

  12. ms82xp | | #12

    Apologies for reviving this old thread, but Chris, how did you end up ducting this?

    I'm in NY state. Looking to install a dehumidifier + ERV and run them in parallel.

    Edit: Attached my "plan". The thicker lines are 8" ducts, the thinner ones are 4"

    1. Jon_R | | #13

      As a general rule, sharing ducts causes flow rate and balance shifts.

    2. Expert Member
      Akos | | #14

      Why would you dump the hot dry air from the dehumidifer into your bedrooms? This is a good way to heat them up.

      The simplest is to duct the dehumidifer to your air handler and the ERV to your rooms. As long as the return from the dining room is large and the dehumidifer+air handler running does not create more than 0.1" of pressure drop there, you can share that duct with the ERV.

      1. ms82xp | | #15

        I don't have an air handler, using mini-splits for air-conditioning.
        I think that I'd be fine with raising the temp in the bedrooms slightly as long as the humidity is ~50%

    3. Jon_R | | #16

      This ERV has practically no change in balance as pressure changes from .1" to .4":

      Looking at a DR90 ducted dehumidifier, flow varies more, but seems to be in spec from 0 to .6".

      Best to calculate the flows, pressure and balance with the various combinations of equipment operating. Also calculate the effect of opening/closing interior doors (3 pascals can cause a large shift in room-to-room airflow balance). Also note that the need for ventilation and dehumidification in a given room/time varies greatly .

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